$\begingroup$Or to irregular distribution, like missing points. You can check if this effect is present with a subset of your sample (first 1000 points, or every 1000th point, or so), and then upload this smaller sample on, e.g., pastebin.com$\endgroup$
– corey979Jan 19 '17 at 18:06

$\begingroup$@corey979 Thank you! I was able to get some part of the image$\endgroup$
– Saesun KimJan 19 '17 at 19:02

Mathematica wants to use some kind of interpolation when making a contour plot, but there's no way to make a decent interpolating function for this. Without fixing your data (preferably at the source), the best you can do is to turn off interpolation altogether

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